1. Field of the Invention
The present invention relates to a linear actuator.
2. The Prior Art
The explanation of the invention departs from electromechanical linear actuators wherein a spindle with a rotationally fixed spindle nut is driven by a reversible electric motor through a transmission. An activation element in the shape of a piston-like tube is attached to the spindle nut. Alternatively, the activation element may be designed integral with the spindle nut cf., e.g., WO 96/12123 to Koch (Okin). The activation element may be brought to move between two end positions, i.e., a retracted position and an expelled position depending on the direction of rotation of the motor. In the two end positions, the actuator is stopped in that the activation element activates an end stop switch, which signals a control device to interrupt the current for the motor of the actuator. Alternatively, the switches function directly as circuit breakers for the current to the motor.
A common way of arranging end stop switches is to mount these on a rail or a strip-type printed circuit board, which is led into a guide in an outer tube which surrounds the activation element. Examples of this type are known from WO 02/29284 to Linak A/S and U.S. Pat. No. 6,513,398 B1 to Dewert Antriebs-und Systemtechnik GmbH & Co. KG. An example of another way of arranging the end stop switches is disclosed in EP 0 647 799 A3 to Linak A/S. Here, the end stop switches are arranged in connection with one end of the outer tube, which is axially displaceable. When the activation element is in its outermost position, it displaces the outer tube a short distance outwards, thus activating the corresponding end stop switch. In the fully retracted position the activation element displaces the outer tube a little inwards, thus activating the other end stop switch. When an end stop switch is activated, it is basically not known whether the activation element is in one or the other end position. Activation of the end stop switch merely causes an interruption of the motor current.
In certain situations it is, however, desired to know whether the activation element is in one or the other end position. This is, e.g., relevant if the actuator and the function which it performs is hidden inside equipment, or if the operation device is located so that neither the actuator nor the function is visible. The existing end stop switches may in principal be part of a detection of in which end position the activation element is located. This is, however, so difficult and expensive in terms of construction that it is not practiced. As an option for linear actuators, systems exist where the position of the activation element may be determined with a high degree of accuracy. This could, e.g., be a potentiometer, a magnetic-based position determination system with a Hall sensor, or an optical system. These systems meet the requirements where it is necessary at all times to know the exact position of the activation element. These types of systems are expensive and exceed the requirements when it is only desired to determine in which of the two end positions the activation element is located.
When the actuators are incorporated into a structure wherein it is required or desired to distinguish between its two end positions, the manufacturer of the machinery was thus obliged to place independent switches in the structure in order to detect whether the activation element is in one or the other end position. This is both difficult and expensive.
The object of the invention is to provide a solution to the outlined problem, where a simple and inexpensive way of detecting whether the activation element is in one of the end positions is desired, and if so which of the end positions.